BESS Working Group J. Drake
Internet-Draft A. Farrel
Intended status: Standards Track E. Rosen
Expires: November 3, 2018 Juniper Networks
K. Patel
Arrcus, Inc.
L. Jalil
Verizon
May 2, 2018
Gateway Auto-Discovery and Route Advertisement for Segment Routing
Enabled Domain Interconnection
draft-ietf-bess-datacenter-gateway-01
Abstract
Data centers have become critical components of the infrastructure
used by network operators to provide services to their customers.
Data centers are attached to the Internet or a backbone network by
gateway routers. One data center typically has more than one gateway
for commercial, load balancing, and resiliency reasons.
Segment routing is a popular protocol mechanism for operating within
a data center, but also for steering traffic that flows between two
data center sites. In order that one data center site may load
balance the traffic it sends to another data center site it needs to
know the complete set of gateway routers at the remote data center,
the points of connection from those gateways to the backbone network,
and the connectivity across the backbone network.
Segment routing may also be operated in other domains, such as access
networks. Those domains also need to be connected across backbone
networks through gateways.
This document defines a mechanism using the BGP Tunnel Encapsulation
attribute to allow each gateway router to advertise the routes to the
prefixes in the segment routing domains to which it provides access,
and also to advertise on behalf of each other gateway to the same
segment routing domain.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 3, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. SR Domain Gateway Auto-Discovery . . . . . . . . . . . . . . 5
3. Relationship to BGP Link State and Egress Peer Engineering . 6
4. Advertising an SR Domain Route Externally . . . . . . . . . . 7
5. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Manageability Considerations . . . . . . . . . . . . . . . . 9
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
Data centers (DCs) have become critical components of the
infrastructure used by network operators to provide services to their
customers. DCs are attached to the Internet or a backbone network by
gateway routers (GWs). One DC typically has more than one GW for
various reasons including commercial preferences, load balancing, and
resiliency against connection of device failure.
Segment routing (SR) [I-D.ietf-spring-segment-routing] is a popular
protocol mechanism for operating within a DC, but also for steering
traffic that flows between two DC sites. In order for an ingress DC
that uses SR to load balance the flows it sends to an egress DC, it
needs to know the complete set of entry nodes (i.e., GWs) for that
egress DC from the backbone network connecting the two DCs. Note
that it is assumed that the connected set of DCs and the backbone
network connecting them are part of the same SR BGP Link State (LS)
instance ([RFC7752] and [I-D.ietf-idr-bgpls-segment-routing-epe]) so
that traffic engineering using SR may be used for these flows.
Segment routing may also be operated in other domains, such as access
networks. Those domains also need to be connected across backbone
networks through gateways.
Suppose that there are two gateways, GW1 and GW2 as shown in
Figure 1, for a given egress segment routing domain and that they
each advertise a route to prefix X which is located within the egress
segment routing domain with each setting itself as next hop. One
might think that the GWs for X could be inferred from the routes'
next hop fields, but typically it is not the case that both routes
get distributed across the backbone: rather only the best route, as
selected by BGP, is distributed. This precludes load balancing flows
across both GWs.
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----------------- ---------------------
| Ingress | | Egress ------ |
| SR Domain | | SR Domain |Prefix| |
| | | | X | |
| | | ------ |
| -- | | --- --- |
| |GW| | | |GW1| |GW2| |
-------++-------- ----+-----------+-+--
| \ | / |
| \ | / |
| -+------------- --------+--------+-- |
| ||PE| ----| |---- |PE| |PE| | |
| | -- |ASBR+------+ASBR| -- -- | |
| | ----| |---- | |
| | | | | |
| | ----| |---- | |
| | AS1 |ASBR+------+ASBR| AS2 | |
| | ----| |---- | |
| --------------- -------------------- |
--+-----------------------------------------------+--
| |PE| |PE| |
| -- AS3 -- |
| |
-----------------------------------------------------
Figure 1: Example Segment Routing Domain Interconnection
The obvious solution to this problem is to use the BGP feature that
allows the advertisement of multiple paths in BGP (known as Add-
Paths) [RFC7911] to ensure that all routes to X get advertised by
BGP. However, even if this is done, the identity of the GWs will be
lost as soon as the routes get distributed through an Autonomous
System Border Router (ASBR) that will set itself to be the next hop.
And if there are multiple Autonomous Systems (ASes) in the backbone,
not only will the next hop change several times, but the Add-Paths
technique will experience scaling issues. This all means that the
Add-Paths approach is limited to SR domains connected over a single
AS.
This document defines a solution that overcomes this limitation and
works equally well with a backbone constructed from one or more ASes.
The solution uses the Tunnel Encapsulation attribute
[I-D.ietf-idr-tunnel-encaps] as follows:
We define a new tunnel type, "SR tunnel". When the GWs to a given
SR domain advertise a route to a prefix X within the SR domain,
they will each include a Tunnel Encapsulation attribute with
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multiple tunnel instances each of type "SR tunnel", one for each
GW, and each containing a Remote Endpoint sub-TLV with that GW's
address.
In other words, each route advertised by any GW identifies all of the
GWs to the same SR domain (see Section 2 for a discussion of how GWs
discover each other). Therefore, even if only one of the routes is
distributed to other ASes, it will not matter how many times the next
hop changes, as the Tunnel Encapsulation attribute (and its remote
endpoint sub-TLVs) will remain unchanged.
To put this in the context of Figure 1, GW1 and GW2 discover each
other as gateways for the egress SR domain. Both GW1 and GW2
advertise themselves as having routes to prefix X. Furthermore, GW1
includes a Tunnel Encapsulation attribute with a tunnel instance of
type "SR tunnel" for itself and another for GW2. Similarly, GW2
includes a Tunnel Encapsulation for itself and another for GW1. The
gateway in the ingress SR domain can now see all possible paths to
the egress SR domain regardless of which route advertisement is
propagated to it, and it can choose one or balance traffic flows as
it sees fit.
The protocol extensions defined in this document are put into the
broader context of SR domain interconnection by
[I-D.farrel-spring-sr-domain-interconnect]. That document shows how
other existing protocol elements may be combined with the extensions
defined in this document to provide a full system.
2. SR Domain Gateway Auto-Discovery
To allow a given SR domain's GWs to auto-discover each other and to
coordinate their operations, the following procedures are
implemented:
o Each GW is configured with an identifier for the SR domain that is
common across all GWs to the domain (i.e., the same identifier is
used by all GWs to the same SR domain) and unique across all SR
domains that are connected (i.e., across all GWs to all SR domains
that are interconnected).
o A route target ([RFC4360]) is attached to each GW's auto-discovery
route and has its value set to the SR domain identifier.
o Each GW constructs an import filtering rule to import any route
that carries a route target with the same SR domain identifier
that the GW itself uses. This means that only these GWs will
import those routes and that all GWs to the same SR domain will
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import each other's routes and will learn (auto-discover) the
current set of active GWs for the SR domain.
The auto-discovery route that each GW advertises consists of the
following:
o An IPv4 or IPv6 NLRI containing one of the GW's loopback addresses
(that is, with AFI/SAFI that is one of 1/1, 2/1, 1/4, or 2/4).
o A Tunnel Encapsulation attribute containing the GW's encapsulation
information, which at a minimum consists of an SR tunnel TLV (type
to be allocated by IANA) with a Remote Endpoint sub-TLV as
specified in [I-D.ietf-idr-tunnel-encaps].
To avoid the side effect of applying the Tunnel Encapsulation
attribute to any packet that is addressed to the GW itself, the GW
SHOULD use a different loopback address for the two cases.
As described in Section 1, each GW will include a Tunnel
Encapsulation attribute for each GW that is active for the SR domain
(including itself), and will include these in every route advertised
externally to the SR domain by each GW. As the current set of active
GWs changes (due to the addition of a new GW or the failure/removal
of an existing GW) each externally advertised route will be re-
advertised with the set of SR tunnel instances reflecting the current
set of active GWs.
If a gateway becomes disconnected from the backbone network, or if
the SR domain operator decides to terminate the gateway's activity,
it withdraws the advertisements described above. This means that
remote gateways at other sites will stop seeing advertisements from
this gateway. It also means that other local gateways at this site
will "unlearn" the removed gateway and stop including a Tunnel
Encapsulation attribute for the removed gateway in their
advertisements.
3. Relationship to BGP Link State and Egress Peer Engineering
When a remote GW receives a route to a prefix X it can use the SR
tunnel instances within the contained Tunnel Encapsulation attribute
to identify the GWs through which X can be reached. It uses this
information to compute SR TE paths across the backbone network
looking at the information advertised to it in SR BGP Link State
(BGP-LS) [I-D.ietf-idr-bgp-ls-segment-routing-ext] and correlated
using the SR domain identity. SR Egress Peer Engineering (EPE)
[I-D.ietf-idr-bgpls-segment-routing-epe] can be used to supplement
the information advertised in the BGP-LS.
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4. Advertising an SR Domain Route Externally
When a packet destined for prefix X is sent on an SR TE path to a GW
for the SR domain containing X, it needs to carry the receiving GW's
label for X such that this label rises to the top of the stack before
the GW completes its processing of the packet. To achieve this we
place a prefix-SID sub-TLV for X in each SR tunnel instance in the
Tunnel Encapsulation attribute in the externally advertised route for
X.
Alternatively, if the GWs for a given SR domain are configured to
allow remote GWs to perform SR TE through that SR domain for a prefix
X, then each GW computes an SR TE path through that SR domain to X
from each of the currently active GWs, and places each in an MPLS
label stack sub-TLV [I-D.ietf-idr-tunnel-encaps] in the SR tunnel
instance for that GW.
5. Encapsulation
If the GWs for a given SR domain are configured to allow remote GWs
to send them a packet in that SR domain's native encapsulation, then
each GW will also include multiple instances of a tunnel TLV for that
native encapsulation in externally advertised routes: one for each GW
and each containing a remote endpoint sub-TLV with that GW's address.
A remote GW may then encapsulate a packet according to the rules
defined via the sub-TLVs included in each of the tunnel TLV
instances.
6. IANA Considerations
IANA maintains a registry called "BGP parameters" with a sub-registry
called "BGP Tunnel Encapsulation Tunnel Types." The registration
policy for this registry is First-Come First-Served.
IANA is requested to assign a codepoint from this sub-registry for
"SR Tunnel". The next available value may be used and reference
should be made to this document.
[[Note: This text is likely to be replaced with a specific code point
value once FCFS allocation has been made.]]
7. Security Considerations
From a protocol point of view, the mechanisms described in this
document can leverage the security mechanisms already defined for
BGP. Further discussion of security considerations for BGP may be
found in the BGP specification itself [RFC4271] and in the security
analysis for BGP [RFC4272]. The original discussion of the use of
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the TCP MD5 signature option to protect BGP sessions is found in
[RFC5925], while [RFC6952] includes an analysis of BGP keying and
authentication issues.
The mechanisms described in this document involve sharing routing or
reachability information between domains: that may mean disclosing
information that is normally contained within a domain. So it needs
to be understood that normal security paradigms based on the
boundaries of domains are weakened. Discussion of these issues with
respect to VPNs can be found in [RFC4364] while [RFC7926] describes
many of the issues associated with the exchange of topology or TE
information between domains.
Particular exposures resulting from this work include:
o Gateways to a domain will know about all other gateways to the
same domain. This feature applies within a domain and so is not a
substantial exposure, but it does mean that if the protocol BGP
exchanges within a domain can be snooped or if a gateway can be
subverted then an attacker may learn the full set of gateways to a
domain. This facilitates more effective attacks on that domain.
o The existence of multiple gateways to a domain becomes more
visible across the backbone and even into remote domains. This
means that an attacker is able to prepare a more comprehensive
attack than exists when only the locally attached backbone network
(e.g., the AS that hosts the domain) can see all of the gateways
to a site.
o A node in a domain that does not have external BGP peering (i.e.,
is not really a domain gateway and cannot speak BGP into the
backbone network) may be able to get itself advertised as a
gateway by letting other genuine gateways discover it (by speaking
BGP to them within the domain) and so may get those genuine
gateways to advertise it as a gateway into the backbone network.
o If it is possible to modify a BGP message within the backbone, it
may be possible to spoof the existence of a gateway. This could
cause traffic to be attracted to a specific node and might result
in black-holing of traffic.
All of the issues in the list above could cause disruption to domain
interconnection, but are not new protocol vulnerabilities so much as
new exposures of information that could be protected against using
existing protocol mechanisms. Furthermore, it is a general
observation that if these attacks are possible then it is highly
likely that far more significant attacks can be made on the routing
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system. It should be noted that BGP peerings are not discovered, but
always arise from explicit configuration.
8. Manageability Considerations
The principal configuration item added by this solution is the
allocation of an SR domain identifier. The same identifier must be
assigned to every GW to the same domain, and each domain must have a
different identifier. This requires coordination probably through a
central management agent.
TBD
9. Acknowledgements
Thanks to Bruno Rijsman for review comments, and to Robert Raszuk for
useful discussions.
10. References
10.1. Normative References
[I-D.ietf-idr-bgpls-segment-routing-epe]
Previdi, S., Filsfils, C., Patel, K., Ray, S., and J.
Dong, "BGP-LS extensions for Segment Routing BGP Egress
Peer Engineering", draft-ietf-idr-bgpls-segment-routing-
epe-15 (work in progress), March 2018.
[I-D.ietf-idr-tunnel-encaps]
Rosen, E., Patel, K., and G. Velde, "The BGP Tunnel
Encapsulation Attribute", draft-ietf-idr-tunnel-encaps-09
(work in progress), February 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, .
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[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, .
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
.
10.2. Informative References
[I-D.farrel-spring-sr-domain-interconnect]
Farrel, A. and J. Drake, "Interconnection of Segment
Routing Domains - Problem Statement and Solution
Landscape", draft-farrel-spring-sr-domain-interconnect-03
(work in progress), January 2018.
[I-D.ietf-idr-bgp-ls-segment-routing-ext]
Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
and M. Chen, "BGP Link-State extensions for Segment
Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-06
(work in progress), April 2018.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-15 (work
in progress), January 2018.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, .
[RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
.
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[RFC7926] Farrel, A., Ed., Drake, J., Bitar, N., Swallow, G.,
Ceccarelli, D., and X. Zhang, "Problem Statement and
Architecture for Information Exchange between
Interconnected Traffic-Engineered Networks", BCP 206,
RFC 7926, DOI 10.17487/RFC7926, July 2016,
.
Authors' Addresses
John Drake
Juniper Networks
Email: jdrake@juniper.net
Adrian Farrel
Juniper Networks
Email: afarrel@juniper.net
Eric Rosen
Juniper Networks
Email: erosen@juniper.net
Keyur Patel
Arrcus, Inc.
Email: keyur@arrcus.com
Luay Jalil
Verizon
Email: luay.jalil@verizon.com
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